Load control devices, such as switches, for example, use mechanical switches, such as electrical relays, to switch alternating currents being supplied to an electrical load. These electrical relays may include at least two contacts (e.g., a fixed contact and a movable contact), and may be in an open state or a closed state. The lifetime of such electrical relays may be shortened by arcs or sparks caused as the contacts of the relay attempt to come into contact with one another (i.e., when the relay attempts to close).
Some electrical loads, such as drivers for light-emitting diode (LED) light sources, behave as capacitive loads. When an LED light source is switched on by the load control device, there is a large in-rush of current into the driver, which quickly subsides as the input capacitance of the driver charges up to line voltage. This temporary current surge can be problematic as the number of drivers controlled by an electrical relay increases. For example, in the case of a full 16-amp (e.g., steady-state) circuit of drivers, the in-rush current can approach 560 amps. Though short-lived (e.g., only a few line cycles or shorter), this level of surge can wreak havoc on the contacts of even a relatively large relay having a high current rating (e.g., 50 amps). The problem stems from the fact that each time a pair of contacts of the electrical relay close or snap together, there is a tendency for the contacts to bounce apart. When this bouncing occurs during a large current surge, the intervening gas or air ionizes and arcing occurs. The arcing has the effect of blasting away the conductive coatings on the relay contacts which eventually causes the relay to fail, either due to erosion of the contact material, or, more commonly, due to welding of the contacts in the closed position.
Some prior art switching circuits for drivers have required advanced components and structures (such as microcontrollers and multiple relays per driver circuit), and complex switching techniques. An example of such a switching circuit is described in greater detail in commonly-assigned U.S. Pat. No. 5,309,068, issued May 3, 1994, entitled TWO RELAY SWITCHING CIRCUIT FOR FLUORESCENT LIGHTING CONTROLLER, and U.S. Pat. No. 5,633,540, issued May 27, 1997, entitled SURGE-RESISTANT RELAY SWITCHING CIRCUIT, the entire disclosures of which are hereby incorporated by reference. Other prior art switching circuits seek to suppress arcs by controlling the relay actuation time such that the relay contact(s) close as nearly as possible to a zero cross of the alternating-current (AC) waveform. An example of such a switching circuit is described in greater detail in commonly-assigned U.S. Patent Application Publication No. 2014/0268474, published Sep. 18, 2014, entitled METHOD OF CLOSING A RELAY SWITCH AND APPARATUS THEREOF, the entire disclosure of which is hereby incorporated by reference. However, switching circuits utilizing these prior art techniques are still susceptible to having stuck relays due to welding of the contacts in the closed position.